Various questions....updating instance, vacuuming, and emails....

1. When you update an instance and tables get new columns, etc. - say
for example you update a subscription class. Is any data lost? Are
tables dropped & readded? I had a situation where I updated my
instance, and some new tables were created called
NS<subscriptionclass>Old. Then they weren't purged during the update.
Then when I updated the instance again, the update failed b/c it was
trying to create these "Old" tables again but they were never dropped
on the first update.
2. Vacuuming - anyone want to give a brief 101 on this? How long
event data stays present, how long till it's purged? I'm assuming it's
a setting you can adjust in the ADF, but haven't looked yet.
3. Should I check the DeviceAddress column in the NSSubscriberDevice
view, via my subscription management, to see if an email address
already exists before I add it again? I'm trying to plan for if
someone registers under accountname1/emailaddress1, and then they come
back and create another account under accountname2/emailaddress1.